In a broadband cellular communication system using spread-spectrum technology, such as code division multiple access (CDMA), the radio spectrum is divided into 40 frequency bands. The 40 bands are divided between a first and a second operator, generally assigning 20 bands to each. The 20 bands are equally divided with 10 bands used for forward communication, with the base station as the transmitter, and 10 bands used for reverse communication, with the mobile radio as the transmitter. Each band is typically 1.288 MHz wide and will handle numerous simultaneous users.
To make and receive calls within such a system, a mobile radio must monitor nearby base sites for paging information and for system information necessary for transmitting access requests on a communication channel referred to as a control channel. The mobile periodically scans for and measures signals of nearby base sites and selects the strongest site (presumably) nearest the mobile. Upon determination of a need to exchange a signal with another mobile radio (or PSTN subscriber) the mobile radio must transmit an access request and receive a channel allocation from the nearby base site (see chpts 1 and 2, Spread Spectrum Systems by Robert C. Dixon, John Wiley & Sons, Inc. 1984).
While CDMA systems provide a substantial measure of noise immunity over conventional narrowband systems, CDMA hand-portable radios typically require a larger, and heavier, power supply than narrowband communication units. The larger battery is necessitated by increased signal processing requirements of CDMA and the digital signal processors (DSPs) and microprocessor units (MPUs) required to process such signals.
The increased power consumption of CDMA signal processing results in limited battery life for CDMA portables. Efforts to increase battery life have included time cycling. For instance, a portable may be activated for monitoring/reception during a first frame followed by sleep cycles for the following nine frames. The process may then be repeated.
Time slotting has been only partially successful because of the time delays required for a CDMA receiver to "acquire" the CDMA signal. A CDMA receiver must remain active for at least two slots (one to acquire the signal and one to decode a communicated signal). Because of the immunity to noise offered by broadband communication systems, a need exists for a more power efficient method of monitoring for paging messages by portable CDMA communication units.